Arterial Blood Gases

Test Overview

An arterial blood gas (ABG) test measures the
acidity (pH) and the levels of oxygen and carbon dioxide in the
blood from an artery. This test is used to check how well your lungs are able
to move oxygen into the blood and remove carbon dioxide from the blood.

As blood passes through your lungs, oxygen moves into the blood while
carbon dioxide moves out of the blood into the lungs. An ABG test uses blood
drawn from an artery, where the oxygen and carbon dioxide levels can be
measured before they enter body tissues. An ABG measures:

Partial pressure of oxygen (PaO2). This measures
the pressure of oxygen dissolved in the blood and how well oxygen is able to
move from the airspace of the lungs into the blood.

Partial pressure of carbon dioxide (PaCO2). This
measures the pressure of carbon dioxide dissolved in the blood and how well carbon
dioxide is able to move out of the body.

pH. The pH measures hydrogen ions (H+) in blood.
The pH of blood is usually between 7.35 and 7.45. A pH of less than 7.0 is
called acid and a pH greater than 7.0 is called basic (alkaline). So blood is
slightly basic.

Bicarbonate (HCO3). Bicarbonate is a chemical
(buffer) that keeps the pH of blood from becoming too acidic or too
basic.

Oxygen content (O2CT) and oxygen saturation (O2Sat) values. O2 content measures the amount of oxygen in the blood.
Oxygen saturation measures how much of the
hemoglobin in the red blood cells is carrying oxygen
(O2).

Blood for an ABG test is taken from an artery. Most
other blood tests are done on a sample of blood taken from a vein, after the
blood has already passed through the body's tissues where the oxygen is used up
and carbon dioxide is produced.

Find out if you need extra oxygen or help with breathing
(mechanical ventilation).

Find out if you are receiving the right
amount of oxygen when you are using oxygen in the hospital.

Measure
the acid-base level in the blood of people who have
heart failure,
kidney failure, uncontrolled
diabetes, sleep disorders, severe infections, or after
a drug overdose.

How To Prepare

Tell your doctor if you:

Have had bleeding problems or take blood thinners, such as
aspirin or warfarin (Coumadin).

Are taking any
medicines.

Are allergic to any medicines, such as those used to
numb the skin (anesthetics).

If you are on oxygen therapy, the oxygen may be turned off
for 20 minutes before the blood test. This is called a "room air" test. If you
can't breathe without the oxygen, the oxygen will not be turned off.

Talk to your doctor about any concerns you have regarding the need for
the test, its risks, how it will be done, or what the results may mean. To help
you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

A sample of blood from an artery is
usually taken from the inside of the wrist (radial artery), but it can also be
taken from an artery in the groin (femoral artery) or on the inside of the
arm above the elbow crease (brachial artery). You will be seated with your arm
extended and your wrist resting on a small pillow. The health professional
drawing the blood may rotate your hand back and forth and feel for a pulse in
your wrist.

A procedure called the
Allen test may be done to ensure that blood flow to your hand is normal. An
arterial blood gas (ABG) test will not be done on an arm used for
dialysis or if there is an infection or inflammation
in the area of the puncture site.

The health professional taking a
sample of your blood will:

Clean the needle site with alcohol. You may be given an injection
of
local anesthetic to numb that area.

Put
the needle into the artery. More than one needle stick may be
needed.

Allow the blood to fill the syringe. Be sure to breathe
normally while your blood is being collected.

Put a gauze pad or cotton ball over the needle site as the needle
is removed.

Put a bandage over the puncture site and apply firm
pressure for 5 to 10 minutes (possibly longer if you take blood-thinning
medicine or have bleeding problems).

How It Feels

Collecting blood from an artery is more
painful than collecting it from a vein because the arteries are deeper and are
protected by nerves.

Most people feel a brief, sharp pain as the
needle to collect the blood sample enters the artery. If you are given a local
anesthetic, you may feel nothing at all from the needle puncture, or you may
feel a brief sting or pinch as the needle goes through the
skin.

You may feel more pain if the person drawing your blood has a
hard time finding your artery, your artery is narrowed, or if you are very
sensitive to pain.

Risks

There is little chance of a problem from having
a blood sample taken from an artery.

You may get a small bruise at the site. You can
lower the chance of bruising by keeping pressure on the site for at least 10
minutes after the needle is removed (longer if you have bleeding problems or
take blood thinners).

You may feel lightheaded, faint, dizzy, or
nauseated while the blood is being drawn from your artery.

Ongoing
bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin
(Coumadin), and other blood-thinning medicines can make bleeding more likely.
If you have bleeding or clotting problems, or if you take blood-thinning
medicine, tell your doctor before your blood sample is taken.

On
rare occasions, the needle may damage a nerve or the artery, causing the artery
to become blocked.

Though problems are rare, be careful with the arm or leg
that had the blood draw. Do not lift or carry objects for about 24 hours after
you have had blood drawn from an artery.

Results

An arterial blood gas (ABG) test measures
the acidity (pH) and levels of oxygen and carbon dioxide in the
blood.

Normal

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab and depend upon the elevation above sea level. Your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

The normal values for children may differ from the adult values listed here.

The concentration of oxygen being breathed, called the
fraction of inhaled oxygen (FiO2), is also usually reported. This is only
useful if you are receiving oxygen therapy from a tank or are on a
ventilator.

Many conditions can change blood gas levels. Your
doctor will talk with you about any abnormal results that may be related to
your symptoms and past health.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include the
following:

You have a fever or an abnormally low body
temperature (hypothermia).

You have a disease that affects how much
oxygen is carried in your blood, such as severe
anemia or
polycythemia.

You smoke just before the
test or breathe secondhand smoke, carbon monoxide, or certain paint or varnish
removers in closed or poorly ventilated areas.

What To Think About

Arterial blood gas (ABG) values alone don't
provide enough information to diagnose a problem. For example, they can't tell
whether low levels are caused by lung or heart problems. Arterial blood gas
values are most helpful when they are reviewed with other examinations and
tests.

An ABG test is often done for a person who is in the
hospital because of severe injury or illness. The test can measure how well the
person's lungs and kidneys are working and how well the body is using energy.

An ABG test may be most useful when a person's breathing rate is
increased or decreased or when the person has very high blood sugar (glucose)
levels, a severe infection, or
heart failure.

If several blood samples
are needed, a thin tube (arterial catheter) may be placed in an artery. Blood
can then be collected when needed.

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